Resistor device and method of making the same



April 1942- G. L. PEARSON 2,280,257

RESISTOR DEVICE AND METHOD OF MAKING THE SAME Filed July 25, 1939INVENTOR By G. LPEARSON G MJMA M A T TORNEV Patented Apr. 21, 1942RESISTOR DEVICE AND METHOD OF MAKING THE SAME Gerald L. Pearson, Towaco,N. J., assignor to Bell Telephone Laboratories, Incorporated, New York,N. Y., a corporation of New York Application July 25, 1939, Serial No.286,307

12 Claims.

This invention relates to resistor devices of the type denoted asindirectly heated resistors and to methods of fabrication thereof. Moreparticularly, it relates to such resistors having a resistance elementthe conductivity of which is highly dependent upon temperatures.

An indirectly heated resistor usually comprises a resistor element, anelectrical heater and an insulating barrier between the resistor elementand the heater. Ordinarily the outside of the unit is also provided withan insulating covering. One of the chief difliculties encountered inmaking indirectly heated resistor units has been the production ofsuitable insulating means, particularly the before-mentioned insulatingbarrier. The insulating means, besides having good insulating propertiesat all operating temperatures, must be a reasonably good conductor ofheat and remain stable over long periods of time when subjected torepeated heating and cooling cycles.

One object of this invention, therefore, is to produce stable,indirectly heated resistor units having insulating means with good heatconducting properties and whose insulation and heat conductingcharacteristics are retained at all operating temperatures.

In accordance with one feature of this invention, the resistor elementand heater of a resistor unit are embedded in a substantially integralmass of insulating material.

Another feature of this invention resides in a resistor having aninsulating portion comprising a glass-like material.

The invention and the foregoing and other features thereof will beunderstood more clearly and fully from the following detaileddescription with reference to the accompanying drawing in which:

Fig. 1 is a sectional view of a resistor unit illustrating one form ofthe invention;

Fig. 2 is a sectional view of a different resistor unit illustratinganother form of the invention; and

Fig. 3 is an elevational view of a resistor unit mounted in a protectiveenvelope, a portion of the envelope being broken away to show theinternal elements more clearly.

In the fabrication of indirectly heated resistors according to thisinvention, a resistor element having firmly attached conductive leads orelectrodes is first fused into a body of glass or glass-like material. Aheater is then applied to the resulting assembly and more glass-likematerial fused therearound. The unit may take several forms two of whichare illustrated in the drawing.

The unit 20 disclosed in Fig. 1 may be made in the following manner: aresistor element, such as a bead ill, of semiconductive material andhaving conductors ll embedded therein, is inserted in a hollow glassbody, such as a glass tube, with portions of the conductors llprojecting from the tube. This assembly is heated sufliciently to softenthe glass. Temperatures from 700 to 900 C. may be employed. A suitablemethod of heating is in a furnace at about 900 C. for thirty seconds inan oxygen atmosphere. The hot glass shrinks around the bead I0 to makegood thermal contact therewith. After cooling, the resulting solid glassbody or rod l2 has the bead l0 firmly embedded therein. In order toinsure good bonding, the glass should have a thermal expansioncoefficient as near that of the bead I 0 and conductors H as possible. Aheater coil l3, preferably prewound, is placed over the glass body orrod l2. The coil may comprise wire of a nickel-chromium alloy or othersuitable heater wire. An oxide-coated wire is preferred. A glass cementI4 is spread over the coil [3 and rod l2 leaving coil ends l5projecting. Upon heating to about 700 C. and cooling the cement forms ahard glassy material which holds coil I3 in place, makes good thermalcontact with the coil l3 and body or rod 12 and offers good electricalinsulation. A preferred cement comprises about 65 per cent of powderedanhydrous sodium borate and about 35 per cent of powdered aluminumoxide, made into a paste by the addition ofsuflicient water. Anotherpreferred cement comprises approximately per cent each of lithium andpotassium borate with water.

Since these units for many purposes are very small, they are preferablymounted in a protective envelope. A glass bulb, such as 22, in Fig. 3 issuitable. The bulb 22 has four supports 23 of conductive material, twoof which are attached to heater leads l5 and two to the resistor leads Hof the unit 20. The bulb is heated, evacuated and sealed off at about 10millimeters of mercury. Evacuation is not necessary for stability butserves to increase the sensitivity of the device by decreasing thethermal loss The encased units may be mounted on suitable bases or thelike adequate for the purpose for which they are employed.

Units 2| of the type shown in Fig. 2 may be constructed as follows: abead-type resistor l0 having conductors H is wetted with a suitableliquid, such as water, and dipped in finely pulverized metal borate.Zinc or potassium borate is preferred. The borate is then fused into aglassy mass l6 surrounding the bead, by the application of heat at from700 to 900 C. The heat may be applied by a flame, oven or other suitablemeans. The glass enclosed head is then inserted into a coil l3 of heaterwire, the assembly again wetted and more metal borate powder applied.Heat is again applied as before, to fuse the borate into a glass. Thewhole assembly is thereby enclosed in a glass bead with the resistorleads II and heater leads I5 projecting there from. Units made in thismanner are ordinarily smaller than those of Fig. 1 for the same sizeresistor element I0. The Wetting steps comprise a simple and convenientmethod of making the borate powder stick until fused and in the case ofassembly with the coil the surface tension centers the inner element inthe heater coil. The unit 2| may be mounted in an envelope in a similarmanner to that employed for unit 20.

The resistor element In may comprise any resistance material havingdesired resistance characteristics. Semiconductive materials which havebeen found suitable for many applications comprise heat-treatedcombinations of metal oxides such as disclosed in application Serial No.274,144 of R. O, Grisdale, filed May 1'7, 1939, now Patent 2,258,646,issued October 14, 1941, and Serial No. 280,692 of E. F. Dearborn, filedJune 23, 1939.

Although the invention has been disclosed by illustrative embodimentsthereof, it is to be understood that various modifications may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims.

What is claimed is:

1. An indirectly-heated resistor unit comprising a head ofsemiconductive material having conductive leads attached thereto, ahollow glass rod fused around and in intimate contact with said bead anda portion of said leads, and a heater coil around said glass rod andheld in contact therewith by a glass-like cementing material fusedaround said coil and rod.

2. An indirectly-heated resistor unitcomprising a head of semiconductivematerial having conductive leads attached thereto, a layer of metalborate fused around and in intimate contact with said bead and a portionof said leads, a heater coil over said layer, and another layer of metalborate fused over "said first layer and coil.

3. An indirectly-heated resistor unit comprising a bead ofsemiconductive material having conductive leads attached thereto, alayer of zinc borate fused around and in intimate contact w th said beadand a portion of said leads,

a heater coil over said layer, and another layer of zinc borate fusedover said first layer and coil.

4. An indirectly-heated resistor unit comprising a bead ofsemiconductive material having conductive leads attached thereto, alayer of potassium borate fused around and in intimate contact with saidbead and a portion of said leads, a heater coil over said layer, andanother layer of potassium .borate fused over said first layer and coil.

5. An indirectly-heated resistor unit comprising a bead of highresistance-temperature coefiicient semiconductive material havingconductive leads attached thereto, a fused glass-like mass enclosing andin intimate contact with said bead and a portion of said leads, and aheater coil around said glass-like mass and held in contact therewith bya layer of fused material comprising sodium borate and aluminum oxide.

6. An indirectly-heated resistor unit comprising a bead of highresistance-temperature coefiicient semiconductive material havingconductive leads attached thereto, a fused glass rod enclos ing and inintimate contact with said bead and a portion of said leads, and aheater coil around said glass rod, and held in contact therewith by alayer of fused material comprising 65 per cent sodium borate and 35 percent aluminum oxide.

7. The method of making a resistor unit that comprises fusing a layer ofglass-like material over a high resistance-temperature coefiicientsemiconductive body and a portion of its attached conducting leads,applying an electrically conductive heater means to said layer, andsecuring said heater means in place by fusing a second layer ofglass-like material over the first layer and the heater means. I

8. The method of making a resistor unit that comprises inserting into aglass tube a body of high resistance-temperature coefficientsemiconductive material having conductive leads attached thereto, saidleads being of sufficient length to project from the tube, heating theassembly to fuse the glass tubing and then cooling to tightly enclosesaid body and leads in the resulting glass body, inserting said glassbody into a. coil of heater wire, applying a cementing material thereto,and heating to fuse the cement into a glass-like mass enclosing the coiland attaching it to the glass body.

9. The method of making a resistor unit that comprises inserting into aglass tube a body of high resistance-temperature coeflicientsemiconductive material having conductive leads attached thereto, saidleads being of sufficient length to project from the tube, heating theassembly at about 900 C. to fuse the glass tubing and then cooling totightly enclose the body and leads in the resulting glass body,inserting said glass body into a coil of heater wire, applying acementing material thereto, and heating at about 700 C. to fuse thecement into a glass-like mass enclosing the coil and attaching it to theglass body.

10. The method of making a resistor unit that comprises wetting asemiconductive body, applying powdered metal borate thereto, heating tofuse the borate into a glass-like layer, applying a heater coil oversaid layer, wetting the assembly, applying more powdered metal borateover the firstlayer and coil, and heating to fuse the borate into aglass-like outer layer.

11. The method of making a resistor unit that comprises wetting asemiconductive body, applying powdered metal borate thereto, heating ata temperature between 700 and 900 C. to fuse the borate into aglass-like layer, applying a.heater coil over said layer, wetting theassembly, applying more powdered metal borate over the first layer andcoil, and heating to a temperature of between 700 and 900 C. to fuse theborate into a glass-like outer layer.

12. An indirectly-heated resistor unit comprising a body ofsemiconductive material having conductive leads attached thereto, a massof glass-like material fused around and in intimate contact with saidbody and a portion of said leads, and a heater coil around saidglass-like mass and held in contact therewith by a glasslike cementingmaterial fused around said coil and mass.

GERALD L. PEARSON.

